The main cellular source of energy for all cell processes is ATP. Most of the cell's ATP is made by H.sup.+ -ATP synthase, a 14 or more subunit protein complex that spans the mitochondrial inner membrane. One subunit is a small, basic ATPase inhibitor protein, designated as IF. IF has been identified and studied in several species, including ox (Walker J. E. et al (1987) Biochemistry 26: 8613-8619) and rat (Higuti T et al (1993) Biochim Biophys Acta 1172: 311-314).
Bovine IF has a 25 amino acid N-terminal signal sequence that is weakly conserved among several other ATPase complex proteins and may direct the proteins into the mitochondrial matrix (Walker et al, supra). Amino acid homologies in a highly conserved region in a central portion of IF may constitute an inhibitory motif that is believed to regulate the activity of ATPase by binding to the C-terminus of the F.sub.1 B-subunit (Polgreen K. E. et al (1995) Biochim Biophys Acta 1229: 176-180). Mammalian IF may exist in two stable conformational forms, a low pH (active) form and a high pH (inactive) form. The interconversion may be due to ionizations of histidine residues in the carboxyl half of the protein resulting in changes in secondary structure (Polgreen et al, supra; Sah J. F. et al (1993) Biochem Biophys Res Comm 194: 1521-1528).
There are strong indications for a role of ATPase regulation in ischemic injury. In animal experiments, blockage of the coronary artery for 10 to 20 minutes was found to result in inhibition of ATPase in cardiac mitochondria (Rouslin W (1983) J Biol Chem 258: 9657-9661). Experimental results on cardiac mitochondria suggest that ATPase may be down regulated during ischemia by IF binding to ATPase (Rouslin W et al (1990) Arch Biochem Biophys 280: 103-111). Genes for components of ATPase are differentially expressed following ischemia-repurfusion in a non-stress specific manner (Knoll R et al (1996) Biochem Biophys Res Commun 221: 402407). Thus, changes in mitochondrial ATPase occur during and after ischemia.
Heart disease and stroke, the most common causes of death in the United States, are ischemic diseases. Ischemic injury can be caused by a gradual narrowing of arteries (arteriosclerosis) or complete blockage by blood clots (thrombosis), leading to tissue disfunction or death. The polynucleotide sequence and polypeptides of novel ATPase inhibitors would satisfy a need in the art by providing a new means for the prevention and treatment of ischemic conditions by regulating ATPase activity.